Abstract: Although the mathematical brain in humans is structurally relatively well-defined, the effects of damage to it can take a wide variety of forms. I discuss some of these and suggest how clinicians can identify them. By contrast, there is one form of congenital disability – usually termed ‘dyscalculia’ – caused by a core deficit in numerosity processing, and I propose that this condition is easy to identify even in kindergarten, and provides a key target for educational intervention.